to the damage of the DNA. As an example, studies of mammalian brain and muscle have shown that DNA repair capability is relatively high during early development when cells are dividing mitotically, but declines substantially as cells enter the post-mitotic state. The effect of reducing expression of DNA repair capability is increased accumulation of DNA damage. This impairs gene transcription and causes the progressive loss of cellular and tissue functions that define aging. As a response to DNA damage, one of the responses triggered by oxidative stress is the activation of the p53. The p53 protein binds to DNA, then stimulates the production of a p21, which is also known as cyclin-dependent kinase inhibitor 1. This ensures that the cell cannot enter the next stage of cell division unless the DNA damage is repaired. However, the p21 cells can trigger apoptosis. Apoptosis or programmed cell death is associated with gradual degradation of the immune system, skeletal muscle, and aging-associated malfunction. Telomere theory of ageing Telomeres are recurring nucleotide sequences that protect the ends of our chromosome; they are sensitive to oxidative stress and degrade during chromosomal replication. Telomerase is a ribonucleotide protein that helps repair and replace degraded telomeres. However, telomerase fails us as we age; it becomes less able to repair telomeres, and our whole body starts falling apart. This means that our cells can no longer divide or divide with errors, and some believe that this contributes to the process of aging. New research has also shown that there is an association between telomere shortening and mitochondrial dysfunction. Nevertheless, over-expression of telomerase increases the chances of cancer. If telomeres stay in repair, there is a greater chance of longevity, but there is also more cell division and a greater chance of mutation, which could result in cancer. Therefore, a long-lived cell is just a time bomb. Enhancing telomerase activity is, therefore, not a solution; it only allows the cells to live longer. Naked mole rats have high telomerase activity, they live long, and were thought by some to never get cancer; and therefore possibly be an exception to this hypothesis. Naked mole rats do get cancer, however. Programmed maintenance theories Theories, such as Weismann's "programmed death" theory, suggest that deterioration and death due to ageing are a purposeful result of an organism's evolved design, and are referred to as theories of programmed ageing or adaptive ageing. The programmed